The present invention relates in general to liquid crystal display devices and, more particularly, to liquid crystal display devices of the active matrix type in which thin-film transistors (TFT) are used. This invention also relates to a method of manufacture of such devices.
Liquid crystal display devices include the in-plane switching (IPS) type of liquid crystal displays, which are designed to apply a lateral electric field to a liquid crystal gap between upper and lower substrates having a layer of liquid crystal material interposed therebetween. The IPS-mode liquid crystal displays are considered to employ a display scheme that is capable of satisfying the requirements for higher image quality, and they have experienced a variety of improvements in image quality.
In IPS-mode liquid crystal display (LCD) devices, a widely used approach is to employ a scheme for switching liquid crystal orientation by use of a lateral electric field which is created between two layers of metal electrodes having an insulating film sandwiched therebetween. However, compared to display devices of the type using twisted nematic (TN) methods, this approach suffers from various problems; for example, it is difficult to enlarge the aperture ratio of picture elements or “pixels”, and, the light utilization efficiency remains low in value. To avoid these problems, a back-light unit that is typically used therein must be able to provide increased luminance. This backlight luminance increase makes it difficult for an LCD module to achieve low power consumption, such as is required for notebook personal computers (PCs) and handheld or “mobile” electronic tools.
In order to solve this problem, a technique has been proposed, for example, in Published Unexamined Japanese Patent Application No. 2002-98982.
Additionally, it is also known that active elements for liquid crystal switching are configured not only from thin-film transistors (TFTs) having their channel regions (switching portions) made of amorphous silicon (noncrystalline silicon), but also from TFTs with channel regions made of polysilicon (polycrystalline silicon).